1. Field of the Invention
The present invention relates to a high carbon steel wire rod with a reduced drawing resistance in a drawing die and superior in wire drawability, in an as-hot-rolled state.
2. Description of the Related Art
As wire rods to be subjected for drawing into very thin wires for use in steel cords or semiconductor cutting saw wires, a high carbon steel wire rod (corresponding to JIS G3502: SWRS72A, SWRS82A) are used that have a carbon content of about 0.7 to 0.8% and a diameter of 5.0 mm or more. If these high carbon steel wire rods are broken in a wire drawing work, the productivity is impaired markedly. To avoid this, the high carbon steel wire rods need to have excellent wire drawability.
Heretofore, for attaining an excellent wire drawability of a high carbon steel wire rod, there has been adopted a method where after hot rolling, the wire rod is cooled with water and then subjected to blast-cooling to make the wire rod structure into a fine pearlite structure, or a method where the wire rod is further subjected to intermediate patenting once or twice before or during a wire drawing process.
Recently, high carbon steel wires have been required to have smaller wire diameters. To meet this requirement and also from the standpoint of improving the productivity, it is desired to provide a direct patenting material or a direct drawing material which permit omission of the patenting treatment before or during wire drawing. To increase the productivity, the high carbon steel wire rods have been increasingly required to have a more excellent breakage resistance and the improved die life.
To meet such requirements, various techniques for improving the wire drawability of the high carbon steel wire rods have been proposed. Japanese Published Examined Patent Application No. 3-60900, for example, proposes a technique to control the tensile strength and the proportion of coarse pearlite, which is recognizable under an optical microscope of 500×, contained in pearlite into appropriate values dependently on C equivalent of a high carbon steel wire rod.
Japanese Patent Application Laid-Open No. 2000-63987 proposes a technique where an average colony diameter of the pearlite structure in a high carbon steel wire rod is set at 150 μm or less and an average lamellar spacing is set at 0.1 to 0.4 μm to thereby improve the wire drawability. Incidentally, the colony indicates a region where lamellar directions of pearlite are regular. The plural colonies form a nodule or a block which is a region where the ferrite crystal orientation is constant. As described in these techniques, the wire rod after hot rolling is produced by adjusting its coiling temperature by water-cooling and then adjusting the amount of blast by a Stelmor adjusting cooler.
According to the technique described in Japanese Published Examined Patent Application No. 3-60900, the die life is improved because a coarse pearlite having a coarse lamellar spacing is present about 10% to 30%. The technique however suffers from insufficient resistance to wire breaking during wire drawing and also an insufficient wire drawability, both required for a direct patenting material or a direct drawing material.
The technique of Japanese Patent Application Laid-Open No. 2000-63987 can improve the die life by making the lamellar spacing somewhat coarser, i.e., to 0.1 to 0.4 μm. Making the lamellar spacing coarser like this results in an average colony diameter of as coarse as 40 μm or more (see its working Examples). This is an insufficient breakage resistance required for a direct patenting material or a direct drawing material.
U.S. Pat. No. 6,783,609 proposes a technique where in order to improve the die life, the lamellar spacing of pearlite is made somewhat wider to decrease the strength of the wire rod, in addition to reducing an average grain diameter of a pearlite nodule which has a physical meaning as a crystal grain to a certain value or smaller. The technique improves the breakage resistance and attains excellent wire drawability even in the case of a pearlite structure having a relatively wide lamellar spacing.
Japanese Patent Application Laid-Open No. 11-302743 proposes a technique to produce a high strength steel wire rod where the breakage resistance is not deteriorated even when the wire rod could be flawed during conveyance with consequent formation of a hard structure subjected to plastic deformation on the steel surface. According to the technique, a high carbon steel wire rod where 70% or more of the structure is pearlite or bainite or a mixture of the two is heated to and retained for 100 seconds or shorter in a temperature of 300° C. to 600° C. before wire drawing, following which the wire rod is cooled by being left as it is or with water.
Japanese Patent Application Laid-Open No. 2001-179325 proposes a technique where a coil is subjected to slow cooling and is softened. The technique however does not intend for use in a direct patenting material or a direct drawing material. Specifically, the technique discloses that the coil cooling rate on a cooling conveyor after hot rolling is controlled by adjusting steel components, austenite grain diameter at the beginning of slow cooling, wire diameter, ring space, and the temperature in a slow cooling.
However, the techniques of U.S. Pat. No. 6,783,609 and Japanese Patent Application Laid-Open No. 11-302743, have no viewpoint of diminishing the drawing resistance of a wire drawing die and improving the wire drawability, and fail in having a sufficient wire drawability required for a direct patenting material or a direct drawing material. Further, simply softening a high carbon steel wire rod after hot rolling (Japanese Patent Application Laid-Open No. 2001-179325) is also insufficient in wire drawability.